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SubjectRe: [tbench regression fixes]: digging out smelly deadmen.
On Sat, 25 Oct 2008, David Miller wrote:

> But note that tbench performance improved a bit in 2.6.25.
> In my tests I noticed a similar effect, but from 2.6.23 to 2.6.24,
> weird.
> Just for the public record here are the numbers I got in my testing.

I have been currently looking at very similarly looking issue. For the
public record, here are the numbers we have been able to come up with so
far (measured with dbench, so the absolute values are slightly different,
but still shows similar pattern)

208.4 MB/sec -- vanilla
201.6 MB/sec -- vanilla
172.9 MB/sec -- vanilla
74.2 MB/sec -- vanilla 2.6.23
46.1 MB/sec -- vanilla
30.6 MB/sec -- vanilla

I.e. huge drop for 2.6.23 (this was with default configs for each
respective kernel).
2.6.23-rc1 shows 80.5 MB/s, i.e. a few % better than final 2.6.23, but
still pretty bad.

I have gone through the commits that went into -rc1 and tried to figure
out which one could be responsible. Here are the numbers:

85.3 MB/s for 2ba2d00363 (just before on-deman readahead has been merged)
82.7 MB/s for 45426812d6 (before cond_resched() has been added into page
187.7 MB/s for c1e4fe711a4 (just before CFS scheduler has been merged)
invalidation code)

So the current bigest suspect is CFS, but I don't have enough numbers yet
to be able to point a finger to it with 100% certainity. Hopefully soon.

Just my $0.02

Jiri Kosina

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